This invention concerns apparatus and methods for scanning and switching charged particle beams with a periodic or quasi-periodic waveform, using a magnetic deflector. The use of magnetic deflectors and electrostatic deflectors for ion beams has been known since the Aston Mass Spectrometer. Both magnetic and electrostatic scanners have been used for scanning electron beams in cathode ray tubes, and methods for controlling non-linearities have been developed. The same scanning methods have been used in ion implanters for scanning ion beams across semiconductor wafers using both two-dimensional electrostatic scanning and one-dimensional magnetic scanning in combination with a one-dimensional xe2x80x9cslowxe2x80x9d translation. Fast switching and scanning magnets must be made of laminated steel, as in a transformer, to minimize losses from eddy currents.
In the Proceedings of 1st conference on radiocarbon dating with accelerators, held at the University of Rochester in 1978, a system was disclosed in which beams of three isotopes of carbon were sequentially switched through an accelerator by switching a potential through which the ions were accelerated upon entering an analyzing magnet, the ions being retarded by an equal amount on leaving the magnet, this is illustrated in FIG. 1a. 
FIG. 1b illustrates a known apparatus designed by the General Ionex Corporation in Massachusetts. FIG. 1b illustrates an apparatus in which beams of three isotopes of carbon were sequentially switched through an accelerator by rapidly changing the field of an analyzing magnet. The magnet was manufactured from laminated steel, which for convenience was cut from commercial transformer laminations, to permit the field to be changed rapidly without incurring eddy current losses.
The Thesis of, D. Phil White, Oxford University, England, 1981, disclosed a functional apparatus for implementing this scheme with a slew rate of 33.75 degrees in 25 microseconds, sequentially switching the isotopes 14C, 13C and 12C through an accelerator with steady periods in between, at repetition frequencies of 10 to 50 Hz. However, it was recognized that the ion-optical properties of this system, although similar, were not identical for the three isotopes.
FIG. 2 illustrates the prior art laminated magnet of U.S. Pat. No. 4,276,477 of Enge. Enge discloses an ion implanter in which an ion beam was slowly scanned across the path of a plurality of targets, which were moved through the beam on a spinning disk. The scanning speed was very slow, around 0.5 Hz. The angles of the scanned trajectories were rendered parallel by a constant shaped-field corrector magnet. The beam passed through an analyzer magnet prior to scanning, and the analyzer magnet was oriented so its dispersive plane was orthogonal to the scanning plane. The resolving aperture was located at a position downstream of the scanner magnet. Because the intended doses were high, and a batch of wafers was implanted (rather than single wafers), the slow scan frequency was acceptable.
FIGS. 3a, 3b and 3c illustrate the prior art system disclosed in U.S. Pat. No. 4,922,106 of Berrian et al. Berrian et al discloses an ion implanter for implanting single wafers in which an analyzed beam is scanned electrostatically at a frequency close to 1 kHz is disclosed. Again the angles in the scanned beam are corrected by a constant shaped-field corrector magnet. The scanning waveform was tailored to achieve a uniform implant.
The present invention addresses the above-identified limitations of conventional systems that magnetically switch or scan or both, a charged particle beam. The present invention provides an approach to enable an ion beam system for use as an ion implanter or a mass spectrometer that quickly and accurately switches between beams of different ion species. The present invention also provides an approach to scan a single ion species quickly at a precisely controlled velocity across a target.
In one embodiment of the present invention, an apparatus is provided for deflecting a charged particle beam by means of a time-varying varying magnetic field emitted by a magnet with a laminated magnetic core and a non-conductive vacuum chamber placed in a gap of the laminated magnetic core. The charged particle beam travels in the vacuum through the time varying magnetic field to switch between beams of different ion species or to scan a single ion species with a controlled velocity across a target. The vacuum chamber includes multiple thin annular sheets of metal or of another conductive material placed within the vacuum chamber to dispose the charged particle beam that passes through holes in each annular sheet without striking the non-conductive vacuum chamber walls. Each annular sheet is electrically connected to each other in a manner that suppresses eddy currents that would otherwise be induced by the time-varying magnetic field.